This invention relates to digital to synchro/resolver converters is general, and more particularly to an improved method of correcting the intrinsic transformation ratio variation versus input angle in such converters.
A digital to synchro/resolver converter is a device which takes an angle represented by a plurality of digital bits and converts it into two or more signals containing angular information. Typically, the signals supplied are at a frequency of 400 Hz and represent the sine and cosine of the digital input angle. If used with a resolver, these outputs are used directly. If to be used with a synchro, the sine and cosine signals are transformed, typically through what is known as a "Scott T" transformer into the three synchro signals which have phases displaced 120.degree. from each other. Hereinafter, when referring to the converter, it will be referred to simply as a digital to synchro converter, that name also implying that it may be used as a digital to resolver converter.
Converters of this nature have been constructed using various types of approximations. Furthermore, because of the repetitive nature of the sine and cosine functions and the fact that certain portions thereof mirror each other, it is typical in such converters to generate sine and cosine values only over an octant or quadrant and to then use selection means to assign the value to either the cosine output or sine output and to give it the proper sign.
A particularly simple and reasonably accurate conversion scheme is disclosed in U.S. Pat. No. 4,072,940, issued Feb. 7, 1978 and assigned to the same assignee as the present invention. The system disclosed therein uses straight line approximations generating slopes and intersects and carrying out the necessary selection and addition thereof to generate the sine and cosine values.
As a general rule, digital to synchro converters which use approximations of this nature have a transformation ratio which is defined as V.sub.o .sqroot. sin .theta..sup.2 +cos .theta..sup.2 /Vr, where V.sub.o is the output at maximum coupling and Vr the input reference voltage. In a perfect converter, this ratio would be a constant. However, in typical converters using an approximation, this ratio varies as much as .+-.7.5% as the input angle changes. These variation result from variations in the term .sqroot.sin .theta..sup.2 +cos .theta..sup.2 which would always be 1 in a perfect converter. The angular accuracy of the converter defined as the ratio sine .theta./cos .theta. is not affected by the transformation ratio variation and is indeed independent of it. In many applications such as use in a null seeking servo loop, the variation gives only second order errors. However, there are many other applications where the transformation ratio does affect the ultimate output. A specific example is where the output of the converter is used to drive a cathode ray rube display. For example, when used in radar displays, if the transformation ratio is constant, a perfect circle will be displayed as the input angle is swept through 360.degree.. If the transformation ratio varies by .+-.7.5%, the display will be grossly distorted, i.e., it will be in the shape of a trapezoid. Since the locus of the circle is the information being transmitted in a radar display, any distortion becomes significant. Empirical data indicates that although variations of .+-.0.25% can still be objectionable, variations within .+-.0.1% cause no discernible distortion.
Methods have been developed for generating improved transformation ratio variation. For example, if the sin .theta. information and cos .theta. information are accurately generated on an absolute basis, the variation will remain very small. However, the means to generate this information on an absolute basis is typically both cumbersome and expensive. One way is through the use of tapped transformers. However, this is done at the expense of size, weight, cost and reliability. This can also be done by generating a large number of linear segments. In order to obtain the required accuracy of .+-.0.1%, the linear segments could not exceed 5.625.degree. in length. However, making the linear segments shorter requires complex switching and resistor networks making the system more costly and complex.
The system disclosed in the aforementioned patent, which uses a linear approximation having segments 11.25.degree., has a transformation ratio variation versus input angle which is .+-.0.25%.
Thus, there is a need for a manner of improving this transformation ratio in a converter of this general nature so as to reduce the transformation ratio variation to .+-.0.1% so as to permit application of a converter of this nature as a cathode ray tube drive.